Photo-polymerizing composites are now used for filling tooth cavities. These composites, once placed within the cavity, are simply exposed to light to harden. This takes a few seconds. However, the cavity must be prepared prior to inserting the photo-sensitive composite. The cavity is first coated with a mixture of a polymer containing calcium hydroxide and a catalyst, in order to protect the nerve of the tooth. Then the cavity surface is coated with an acid for etching said surface, so as to improve the fastening of the filler to the tooth. Then a rather liquid resin material is coated on the cavity surface, in order to make a firm bond between the filler and the tooth. For the last two operations, a conventional method is as follows: the acid and the resin are each enclosed in a small container with a screw cap to prevent evaporation of the treating acid and resin. The dentist must unscrew the cap of the related container, empties a few drops of the treating liquid on a platen and then press a cotton swab onto the liquid on the platen by means of a dentist pliers holding the swab. Then the bottle must again be closed. These are time-consuming operations, especially because two similar treatments must be made for each tooth filling.